FIGS. 1-3 on the appended drawing illustrate different steps of a known method of fabricating an emitter-base portion of a bipolar double-poly transistor, such as described in US 2001/0012655A1.
It should be noted that only half of the emitter window of the transistor structure is shown.
In a manner known per se, the transistor structure illustrated in FIG. 1 comprises a layer 1 of base silicon that has been deposited on a silicon substrate 2, and a layer 3 of silicon dioxide that has been deposited on the base silicon layer 1.
An etched-out emitter window 4 extends through the silicon dioxide layer 3 and the base silicon layer 1 down into an etched-out portion of the silicon substrate 2.
Also in a manner known per se, a layer 5 of silicon dioxide has been formed, e. g. thermally grown, in the emitter window 4. The silicon dioxide layer 5 covers the walls of the emitter window 4 and the bottom of the emitter window 4, i. e. the etched-out portion of the silicon substrate 2.
Moreover, in a manner known per se, silicon nitride spacers 6, only one of which is shown in FIG. 1, have been formed on the silicon dioxide layer 5 in the emitter window 4, e. g. by deposition of silicon nitride and reactive ion etching.
In FIG. 2, the part of the silicon dioxide layer 5 that is not covered by the spacer 6 at the bottom of the emitter window 4 has been etched away, e. g. by a wet-etching procedure using hydrofluoric acid in a bath or vapor. However, also part of the silicon dioxide layer 5 that lies under the spacer 6 at the bottom of the emitter window 4 will be etched away as indicated by 7.
In the same wet-etching step, part of the silicon dioxide layer 3 facing the emitter window 4 is etched away as well as part of the silicon dioxide layer 5 between the base silicon layer 1 and the spacer 6. Thus, a narrow slot 8 is formed, that extends down between the base silicon layer 1 and the spacer 6.
After cleaning of exposed silicon surfaces, which includes removal of native silicon dioxide by exposure to hydrofluoric acid in a bath or vapor, emitter silicon 9 is deposited into the emitter window 4. The emitter silicon 9 that is deposited into the emitter window 4 will fill the slot 8 and hereby come into direct contact with the base silicon layer 1 as indicated within an encircled area 10 in FIG. 3.
Thus, the emitter silicon 9 will be short-circuited to the base silicon layer 1 making the transistor inoperative.
It would of course be possible to etch away the part of the silicon dioxide layer 5 that is not covered by the spacer 6 at the bottom of the emitter window 4 in FIG. 2 by a dry-etching procedure. Hereby, the slot 8 would not be formed. However, when the silicon surfaces are exposed to hydrofluoric acid in a bath or vapor to remove native silicon dioxide as a part of a cleaning step immediately prior to emitter silicon deposition, the slot 8 will be formed. This cleaning step cannot be carried out by a dry-etch process but has to be carried out by exposing the silicon surfaces to hydrofluoric acid.